Fan Beam CT
In a typical computed tomography system, an x-ray source, mounted to a rotating gantry, is collimated to form a fan beam with a defined fan beam angle. The fan beam is typically oriented to lie within the "gantry plane" normal to the axis of rotation of the gantry, and is transmitted through an imaged object to an x-ray detector array also oriented within the gantry plane. The axis of rotation of the gantry is also referred to as the z-axis,
The detector array is comprised of a line of detector elements, each of which measures the intensity of transmitted radiation along a ray projected from the x-ray source to the particular detector element. The intensity of the transmitted radiation is dependent on the attenuation of the x-ray beam along that ray by the imaged object.
The x-ray source and detector array may be rotated on the gantry within the gantry plane and around a center of rotation so that the "gantry angle" at which the fan beam axis intersects the imaged object may be changed. At each gantry angle, a projection is acquired comprised of the collected intensity signals from each detector element. The gantry is then rotated to a new angle and the process is repeated to collect projections data along a number of gantry angles to form a tomographic projection set.
Often, 2.tau. radians or 360.degree. of gantry rotation will be used to collect the projection set; however, for fan beam CT systems, it has been determined that a mathematically complete projection set may be obtained with as little as .tau. radians, plus the angle of the fan beam of gantry rotation. The use of less than 2.tau. radians of gantry rotation to collect a projection set will be referred to generally as "half scan".
The acquired tomographic projection sets are typically stored in numerical form for later computer processing to "reconstruct" a slice image according to reconstruction algorithms known in the art. A projection set of fan beam projections may be reconstructed directly into an image by means of fan beam reconstruction techniques, or the intensity data of the projections may be sorted into parallel beams and reconstructed according to parallel beam reconstruction techniques. The reconstructed tomographic images may be displayed on a conventional CRT tube or may be converted to a film record by means of a computer controlled camera.
A typical computed tomographic study involves the acquisition of a series of "slices" of an imaged object, each slice parallel to the gantry plane and having a slice thickness dictated by the size of the focal spot, the width of the detector array, the collimation, and the geometry of the system. Each successive slice is displaced incrementally along a z-axis, perpendicular to the x and y axes, so as to provide a third spatial dimension of information. A radiologist may visualize this third dimension by viewing the slice images in order of position along the z-axis. Alternatively, the numerical data comprising the set of slices can be processed further by computer to produce slice images at any orientation through the image object or to produce three dimension perspective representations of the image object.